A signal travelling along an unshielded copper twisted pair undergoes frequency-dependent attenuation due to the transmission characteristics of the cable. This distortion can be modelled as a slowly time-varying filtering operation applied to the transmitted signal. The relative variations in the attenuation at different frequencies cause phase shifts in the transmitted waveform and, in digital transmission systems, ultimately lead to spreading of the transmitted symbols into adjacent symbol intervals, a phenomenon known as inter-symbol interference (ISI). This results in the receiver committing errors when data decisions are made based on the distorted received signal.
In order to compensate for the distortive effects of cable loss, it is customary to employ a linear equalizer, possibly even an adaptive one, followed by a decision-feedback equalizer (DFE). The linear equalizer usually consists of a filter, either digital or analog, which has a frequency response equal to (or close to) the inverse response of the twisted pair "channel". Therefore, if the transmitted signal is attenuated by a downwards sloping frequency response as it travels along the twisted pair, then the frequency response of the linear equalizer will have an upwards sloping shape.
The purpose of linear equalization is to equalize, or "flatten", the overall channel response affecting the transmitted signal as it arrives at the decision-feedback equalizer. This linear equalization process has the effect of removing much, but not all, of the ISI corrupting the transmitted signal. The DFE is then used for removing any remaining ISI and for making data decisions on the received digital signal. Much theory has been developed around the problems of channel equalization and decision-feedback equalization, and various implementations have proven successful in the case of a digital signal transmitted along isolated media.
However, when a telephone company considers delivering digital signals to a plurality of subscribers over a copper twisted pair infrastructure, it is often the case that multiple twisted pairs are bundled together for at least part of the journey between a central office and the subscribers. As a result of poor shielding provided by the thin layer of insulant surrounding each copper wire, electromagnetic fields may be induced by one wire into other wires in the bundle, creating an effect known as crosstalk.
In general, crosstalk couples more at higher frequencies and therefore after a long voyage along a twisted pair, the effect of crosstalk on a transmitted signal will be most noticeable at high frequencies. A serious problem then occurs if the linear equalizer in the receiver boosts the high frequencies in an attempt to equalize the channel: the effects of crosstalk noise, which is stronger at higher frequencies, are actually enhanced.
At the DFE, therefore, the received signal equalized in accordance with prior art techniques has reduced ISI but possibly increased crosstalk noise. When crosstalk noise becomes the dominant cause of distortion, as when many twisted pairs are bundled together, the decision-feedback equalizer is incapable of making correct data decisions, with obvious deleterious consequences.